Purification of perfluorochlorocarbons



United States Patent 3,218,363 PURIFICATION OF PERFLUOROCHLORO- CARBONSRobert N. Haszeldine, Disley, England, assignor to Pennsalt ChemicalsCorporation, Philadelphia, Pa., a corporation of Pennsylvania NoDrawing. Original application- Aug. 14, 1957, Ser. No. 678,025, nowPatent No. 3,004,908. Divided and this application Aug. 30, 1961, Ser.No. 134,822 Claims priority, application Great Britain, Aug. 15, 1956,24,208/56 6 Claims. (Cl. 260-648) This application is a division ofapplication Serial No. 678,025, filed August 14, 1957,, now US. PatentNo. 3,004,908.

This invention relates to a method of purifying perhalocarbon compounds,and in particular perfluorocarbon compounds.

Perfluorocarbons are saturated compounds which contain only carbon andfluorine. Such compounds are extremely inert, both to thermal breakdownand to chemical attack. They are diflicult to oxidize, and are stable toconcentrated acids, concentrated alkalis, and normal oxidizing andreducing agents. They will, in fact, react with the alkali metals onlyat high temperatures. This marked thermal and chemical stability hasmade fluorocarbons highly useful in the fields of heat transfer media,dielectrics, fire-extinguishers, high temperature coolants, lubricants,hydraulic fluids, and special solvents. Compounds containing onlyfluorine, carbon and chlorine are also extremely inert and similarlyuseful when the major portion of the halogen is fluorine.

One major difliculty in the use of perfluorocarbons has been the factthat, as normally prepared, perfluorocar bons contain small amounts ofresidual hydrogen. The

normal commercial methods for perfluorocarbon preparation are (a) thedirect reaction of fluorine with a hydrocarbon in the presence of asilver or gold catalyst and nitrogen as diluent, and (b) the passage ofa hydrocarbon in th vapor phase over a reactive metal fluoride such ascobaltic trifluoride, manganic fluoride, silver difluoride, or leadtetrafluoride. By either of these methods the hydrocarbon is convertedinto the corresponding perfluorocarbon. Breakdown of the carbon chainoccurs to a certain extent, and fluorine-containing compounds of shorterchain length are thus always produced simultaneously. Such compoundsusually can be separated by simple distillation, but it is much moredifficult to separate from the desired perfluorocarbon the materialwhich is almost completely fluorinated but which still contains smallamounts of residual hydrogen. The amount of the hydrogen left is oftenless than one atom of hydrogen per molecule of fluoro-compound, on theaverage; but the presence of the hydrogen can be detected by sensitivetechniques such as infra-red 'or nuclear magnetic resonancespectroscopy, and by analysis.

Hydrofluorocarbon impurities are undesirable in both perfluorocarbonsand perfluorochlorocarbons for many uses, since on standing they slowlyliberate hydrogen fluoride, and this will etch any glass apparatusnearby,

or will combine with the metal of any apparatus in which theperfluorocarbon or perfluorochlorocarbon is contained. The liberation ofhydrogen fluoride is accelerated by heat, and by the presence of metal,withthe result that the presence of hydrofluorocarbon inpurities leadsto corrosion problems such as militate against the use ofperfluorocarbons and perfluorochlorocarbons. Even less than 0.l0.2% ofhydrogen in the perfluorocarbon is undesirable. Although pureperfluorocarbons are extremely inert chemically, and as such have highly"Ice desirable properties, hydrogen-containing perfluorocarbons are muchmore reactive chemically, mainly as a result of the splitting outhydrogen fluoride to give a reactive olefin. Strong alkalis, forexample, will bake a hydrogen-containing perfluorocarbon darker, and itis easily shown that fluoride is steadily liberated. This reaction isnot specific to every hydrogen atom present in the molecule, however,and mere treatment with alkali is insuflicient to give adequatepurification of the perfluorocarbon. Even after treatment with strongalkali for several days, which removes some of the hydrogencontainingimpurity, the semi-purified perfluorocarbon will still liberatehydro-gen fluoride on standing, and particularly when heated in thepresence of metal.

There is no easy method known for the removal of hydrogen-containingimpurities to give a perfluorocarbon which shows the correct desirableproperties.

Som of the hydrogen-containing impurity in perfluorocarbons can beremoved by laborious fractional distillation, but the spread of boilingpoints when hydrogen is replaced by fluorine in an organic compound isusually not wide, and azeotropic formation is prevalent. It is thusalmost invariably found that a perfluorocarbonprepared by either of thetwo methods described above is contaminated by hydrogen-containingmaterial even after long distillation.

A second procedure for the removal of hydrofluorocarbons is clearly torecycle the slightly impure perfluorocarbon through the apparatus usedto prepare it, in the hope that the residual hydrogen would be replacedby further contact with the fluorinating agent. This will indeed improvethe quality of the crude perfluorocarbon, but such recycling treatmentsare expensive and laborious, and still do not remov completely thehydrogencontaining impurities; furthermore, some breakdown usuallyoccurs during recycling, with the result that some of the desiredperfluorocarbon product it lost. Even when such a recycling procedure isrepeated several times, traces of residual hydrogen can always be foundin the perfluorocarbon products.

The present invention is concerned with a new method for thepurification of perfluorocarbons by removal from such compounds of thesmall percentage of saturated polyfluoro-compounds which contain a smallamount of hydrogen. The invention enables perfluorocarbons to beobtained with a hydrogen content of 0.01% or less and suchperfluorocarbons can be obtained from the less pure perfluorocarbonswhich have been subjected to a known purification procedure such as ismentioned above. The invention is also applicable to the purification ofperfluorochlorocarbons to remove hydrogen-containing materials.

It is, therefore, a primary object of this invention to provide a methodof purifying perhalocarbon compounds, in which at least the majorportion of the halogen 'constituents is fluorine and the remainder ischlorine, from impurities containing hydrogen, to provide a purifiedmaterial having an exceedingly low hydrogen content.

Another object of this invention is to provide a practical method forobtaining extremely pure perfluorocarbon and perfluorochlorocarboncompounds.

In accordance with the present invention perhalocarbon compounds freefrom bromine and iodine are purified by subjecting the impure compoundto severe oxidizing conditions and removing the products of oxidation.By severe oxidizing conditions is meant the use of an oxidiz ing agenthaving a high enough oxidation potential combined with a sufficienttemperature or otherreaction con ditions to oxidize compounds containingC'H groups.

As used herein, the terms perhalocarb'on compound and perhalocarbon meana compound containing only carbon and halogen, the terms perfiuorocarboncompound and perfluorocarbon mean a compound containing only fluorineandcarbon, and the terms perfluorochlorocarbon compound andperfluorochlorocarbon mean a compound containing only fluorine, chlorineand carbon. The perfluorochlorocarbon to which this invention isapplicable are those having an atomic ratio of fluorine to chlorine ofat least one.

By a polyfluorohydrocarbon group is meant a group containing mor thanone atom of fluorine; hydrogen and carbon. By apolyfluorochlorohydrocarbon group is meant a group containing more thanone atom of fluorine, more than one atom of chlorine; hydrogen andcarbon.

In general, the invention is applicable in purification of compounds ofthe above nature regardless of the number of carbon atoms, andfluorocarbon oils consisting of a chain CF CFCI units have beensuccessfully purified by the method in this invention as well ascompounds having comparatively low molecular weight.

The purification method which is th subject of the present inventioninvolves, in essence, the subjection of the hydrogen-containingimpurities in crude perfluorocarbons or perfluorochlorocarbons to freeradical oxidation whereby such impurities undergo selective degradationto yield readily separable products. Free radical oxidation may bedefined as the reaction of the free radical with oxygen to providecarbon-oxygen bonds.

Preferably, an initiator is used to provide free radicals from theimpurities although it is possible to achieve the desired oxidationwithout such an initiator under reaction conditions sufficient toprovide some free radical formation such as ultra-violet irradiation orthermal activation.

In its broad aspect, any initiator capable of abstracting hydrogen toform a free radical may be used. However, in a preferred form, themethod involves subjecting, either in the liquid or gas phase, the crudeperhalocarbon to the action of an energy source such as ultravioletlight or heat in the presence of a halogen type initiator and oxygen,whereby the halogen atoms so produced abstract hydrogen from theimpurities to give free radicals which react extremely rapidly with theoxygen to give chain degradation, i.e. fission of carbon-carbon bonds togive ultimately carbonyl fluoride. By halogen type initiator is meant amember selected from the class consisting of halogen other than iodineand interhalogen compounds other than iodine fluorides. Examples ofhalogen type initiators that may be used are chlorine, bromine, fluorineother than iodine, and interhalogen compounds such as ICl, C113,, C115,IBr and BrF The preferred interhalogen compounds are halogen fluorides,other than iodine fluorides. Examples of peroxide compounds which may beused as initiators are benzoyl peroxide, acetyl peroxide,hexachloroacetyl peroxide, hexafluoroacetyl peroxide and di-tertbutylperoxide, and examples of azo-compounds which may be used as initiatorsare alpha, alpha-azo-di-isobutryronitrile and the diazo compounds suchas diazomethane. As stated above, the use of an initiator may bedispensed with altogther. However, except in special cases, the use ofan initiator will be preferred since the reaction is thereby speeded upand its efliciency improved.

In general, the reaction is carried out under favorable conditions forthe formation of free radicals and oxidation thereof. Preferablyphotochemical energy is used; however, other activation conditions suchas thermal energy, X-rays, radioactive material energy sources,irradiation by high energy particles, and exposure to strong electronfields may also be used.

Although molecular oxygen is the preferred oxidizing agent, other agentscapable of effecting the requisite chain degradation may also beemployed. Thus, for example, the free radical oxidation treatment may becarried out with ozone, oxides of nitrogen or indeed any compound whichunder the reaction conditions employed will dccompose to give oxygen.

Any of the above-described methods if initiation of the reaction, or acombination of them, is suflicent to :break the carbon-hydrogen bond inthe undesired impurity to produce the required free radical which canthen react with oxygen by chain degradation to give carbonyl fluoride asthe end product or carbonyl halide such as carbonyl chloride andcarbonyl fluorochloride when perfluorochlorocarbons are being purified.The pure perhalocarbons, i.e. compounds which contain no hydrogen, areconsiderably more stable to free radical oxidation than are the.impurities which contain small amounts of hydrogen, and consequentlyonly slight breakdown occurs of the pure perhalocarbon during thetreatment. The carbonyl halide, together with smaller amounts of acylhalide produced by the breakdown process, are readily removed from thepure perhocarbon :by distillation or by washing with water or aqueousbase.

In the preferred embodiment of performing the purely thermal process,the crude perhalocarbon is first mixed with oxygen and with a suitableinitiator such as fluorine, chlorine, bromine, or a compound capable ofgiving an alykl radical and then passed through a furnace wherein chaindegradation of the impurity occurs. This chain degradation can bevisualized as involving reactions of the following types: I

Initiator Oz etc.

(n 1) C 0 F2 heat light l lo.

...0 F .C F .O F ...C F 0 Fz.0 further breakdown acyl fluoride re- C 0moved by water washinglater .CFz.CFz...

C 0 F ultimately C O F g ultimately Olefinic impurity is also removed,e.g.

F F 03 (g cCuFro COFg-I- .(CFQH.

It follows, therefore, that the free radical oxidation proceduredescribed herein is also applicable to the purification of fluorocarbonscontaminated with olefinic substances instead of, or as well as, thehydrogen-containing impurities discussed above. Also, it is obvious that016- finic materials are removed during the purification process forremoval of hydrogen.

Of particular note is the fact that perfluorocarbon oils can besubstantially freed from hydrogen by the method of the invention, as canperfluorocholocarbon oils containing a high percentage of fluorine withsmall amounts of hydrogen in the end groups. The products so obtainedare thermally more stable, are more stable when kept at roomtemperature, and do not liberate acid fumes, or tarnish metals.

The process can be applied to straight chain perfluorocarbons, e.g. C Fand to monoand polycyclic perfiuorocarbons with or without side chains.

The oxidation reaction can be carried out under pressure, e.g. up to 10atmospheres, under atmospheric pressure, or under sub-atmosphericpressure down to 0.1 mm. The reaction temperature may vary widely butwill usually be in the range from -500 C. Conventional glass, silica, ormetal apparatus can be used, with either flow or batch processes.

The preferred process operates with light as the activating energysource, particularly light of wave length 3500 A. and chlorine orfluorine as initiators, with oxygen in excess (ratio halogenzoxygen ca.1:5 to 1:100), at a pressure between 100 mm. and atmospheres.

The reaction time required to efiect the requisite degree ofpurification will of course depend on the reaction conditions employedand the nature and amount of impurities present. Completion ofpurification may, however, readily :be determined by observation of thepercent transmission to ultraviolet light in the 20002300 A. region ofthe spectrum. A pure perfluorocarbon absorbs only weakly in this regionof the spectrum, whereas a compound containing hydrogen such as ahydrocarbon absorbs relatively strongly. Removal of C-H groups from acrude perfluorocarbon thus causes an increase in the percenttransmission in this region. A cell of long path length enhances thesensitivity. A more sensitive method for detection of C-H impurity isinfrared spectroscopy using a liquid sample in a cell of ,long pathlength. The characteristic C-H vibrations are clearly apparent in thecrude perfluorocarbon and diminish as purification proceeds. The amountof hydrogen in the molecule can also be determined by use of theprocedure of Anal. Chem., 1947, 11, 146. Preferably purification iscontinued until the hydrogen content is reduced to less than 0.01%,preferably, e.g., less than 0.004%.

When heat is used as the energy source, as disclosed in Examples 17-20,the reaction will generally be carried out in the temperature range350-700 C. with a molecular ratio of initiator to oxygen ranging from1:5 to 1:100. The contact time will generally be within the range offrom one second to minutes chosen so that the impurity is reduced to thedesired low value when the perhalocarbon is tested in accordance withthe methods disclosed in the previous paragraph. When the time islimited in accordance with the procedures of the preceding paragraph,self-pyrolysis of the pure fluorocarbon isinsignificant. When thetemperature is in the range 350-500 C. the contact time will generallybe within the range 15 seconds to 10 minutes; when the temperature is inthe range 500-700 C., the contact time will generally be within therange 1 to 15 seconds.

The invention will be further described with reference to the followingspecific examples, it being understood that these examples are given forpurposes of illustration only and are not to be taken as in any waylimiting the invention beyond the scope of the appended claims.

6 Example 1 A commercial specimen of perfluorodimethylcyclohexane wasfound to contain 0.07% hydrogen. It slowly liberated fluoride when keptin contact with aqueous or alcoholic KOH or piperidine, and the mixturedarkened in colour. Passage of the C F over COF at 300 caused somepurification, but the percent hydrogen did not decrease below 0.05, andfurthermore, losses of C F by breakdown and by manipulation lossesamounted to 10%. The hydrogen content was reduced to 0.05% by prolongedreaction with concentrated aqueous KOH, followed by chromic acid, butthe procedure was long and tedious.

A sample of the perfluoromethylcyclohexane (20.0 g.) was sealed in a 200ml. silica tube with chlorine (0.5 g.) and oxygen (5 atmos. pressure)and the tube was shaken at room temperature whilst exposed toultraviolet irradiation from a powerful lamp source. After 48 hours(probably excessive) the tube was opened and the more volatile productsremoved. After removal of the excess of chlorine by treatment withmercury, the gaseous products from several such experiments werecombined and examined spectroscopically and found to be carbonylfluoride, a very small amount of carbonyl chlorofluoride, and a smallamount of silicon tetrafluoride arising by attack of COF on the wall ofthe reaction vessel. The liquid product from this experiment was washedwith aqueous alkali and distilled in vacuo to give purified C F 19.0 g.,containing 0.0020.005% hydrogen. The pure fluorocarbon did not liberatefluoride when kept .in contact with aqueous alkali or with an organicbase, and was free from chloro compounds. The course of the purificationof the C F could be followed by any of the methods described earlier.

Example 2 Crude perfluorodimethylcyclohexane (20 g.) containing 0.05hydrogen was sealed in a 200-ml. silica tube with chlorine (1.0 g.) andoxygen (7 atmos. pressure) and heated to whilst exposed to visible light.from four filament light bulbs. After 4 days the contents of the tubewere worked up as in Example 1. The purified 0 1 (18.2 g.) contained0.01% hydrogen, and liberated fluoride ion only on contact with conc.aqueous alkali for several weeks.

Example 3 Crude perfluorodimethylcyclohexane (20 g.) containing 0.05%hydrogen was mixed in a 20-1. bulb with chlorine (0.8 g.) and oxygen toa pressure of 700 mm. The vapor was exposed to ultraviolet light from an'internal lamp for 24 hr. whilst theflask was slowly shaken. Theproducts were pumped from the bulb and washed with dilute aqueous alkaliand then were redistilled in vacuo to give purified C F (18.8 g.)containing 0.005- 0.007% hydrogen. The aqueous alkaline solution gave apositive test for fluoride.

Example 4 Crude perfluoroheptane (15 g.) containing 0.10% hydrogen wassealed with fluorine (1.0 g.) and oxygen (8 atmos.) in a 250-ml. silicatube and irradiated with ultraviolet light for 10 hrs. The liquidreaction products were distilled to give purified C7F16 containing lessthan 0.005% hydrogen. A control experiment showed that fluorine in theabsence of oxygen reacted only relatively slowly with the crude C FExample 5 Perfluoroheptane (12.0 g.) containing 0.08% hydrogen wassealed with chlorine trifluoride (0.5 g.) and oxygen (3 atmos.) in asilica flask and exposed to ultraviolet light for 14 hr. After removalof the more volatile products by treatment with dilute aqueous sodiumhydroxide, the residual liquid was distilled in vacuo to 7 give purifiedC7F15 (11.1 g.) containing less than 0.004% hydrogen. Example 6 Aparallel experiment to Example using chlorine monofluoride instead ofchlorine trifluoride gave purified C F containing 0.004% hydrogen.

Example 7 Perfluoro 1:3:S-trimethylcyclohexane (5 ml.) containing 0.09%hydrogen was sealed with bromine trifluoride (0.3 g.) and oxygen atmos.)in a 150-ml. silica tube and irradiated with ultraviolet light for 19hr. Distillation in vacuo gave purified C F containing less than 0.004%hydrogen.

Example 8 Pentadecafluoro-dimethylcyclohexane, C F H (2.1 g.) was sealedwith chlorine (0.5 g.) and oxygen (3 atmos.) and exposed to intenseultraviolet light for 30 hr. to give unchangedpentadecafluoro-dimethylcyclohexane (0.9 g.) and carbonyl fluorideidentified spectroscopically. Irradiation for a further 2 days resultedin complete destruction of the C l- H, thus showing that even relativelystable hydrogen-containing fluorocarbons are susceptible tohalogensensitized photochemical oxidation.

Example 9 A crude fluorocarbon C F obtained by vapor phase reaction ofnaphthalene with cobalt trifluoride contained 0.05% hydrogen. A specimenof it (6.3 g.) sealed with chlorine and oxygen (3 atmos.) and exposed tointense ultraviolet light for 24 hr. gave purified C F containing 0.005%hydrogen. The purified specimen was resealed with chlorine and oxygenand irradiated for a further 4 days. The hydrogen content of thespecimen was now less than 0.002%, and could not be determined by theanalytical techniques available.

Example 10 A 15-ml. sample of perfluoro-methylcyclohexane which had beenpurified by prolonged contact with concentrated aqueous potassiumhydroxide followed by refluxing with concentrated chromic acid, aqueousalkaline perman-,

ganate, and finally cyclohexylamine contained 0.006% hydrogen. It wassealed with oxygen (4 atmos.) and a mixture (0.3 g.) of chlorine (80%)and chlorine monofluoride and strongly irradiated for 36 hr. withultraviolet light. After removal of free halogen and readilyhydrolysable products in the usual way, the residual liquid wasfractionated to give purified 0 F containing less than 0.003% hydrogen.

Example 11 A fluorochloro-oil B.P. 150-250/ 1 mm. consisting of a chainof {CF CFCI} units with perhalogenalkyl end groups was contaminated bysmall amounts of hydrogencontaining impurities which could not beseparated by distillation, solvent extraction, or treatment with aqueousalkali without substantial loss of material. The hydrogen content was0.08%. It was diluted with purified C F containing 0.004% hydrogen andsealed with chlorine (1 g.) and oxygen (3 atmos.) then irradiated for 24hr. with ultraviolet light. Removal of the solvent and redistillationgave a purified oil which contained 0.006% hydrogen. The original sampleof the oil liberated acid fumes when kept for 3 months in a Pyrex bottleand tarnished aluminum, steel and nickel wires immersed in it; thepurified sample was water-white and free from acid fumes when similarlystored, and there was no attack on immersed aluminum, steel and nickelwires.

Example 12 Purified perfluorodimethylcyclohexane containing less than0.002% hydrogen was sealed with chlorine (1 g.) and oxygen (2 atmos.) ina silica vessel and exposed to ultra- 8 violet light for 4 weeks.Distillation in vacuo gave a 96% recovery of the C F thus showing that apurified perfluorocarbon is relatively stable under the conditionsnormally used for removal of hydrogen-containing impurities.

Example 13 A crude fraction (3.1 g.) from the vapor phase fiuorinationof toluene with fluorine in presence of a goldplated copper catalyst hadan average composition and contained olefinic material. It was mixedwith chlorine (1 g.) and oxygen to a pressure of 690 mm. in a 20-1.flask and irradiated by ultraviolet light for 2 days. Only 0.47 g. ofmaterial remained which was substantially free from hydrogen (0.009%)and contained no olefinic impurity, and consisted of a mixture ofperfluoromethylcyclohexane C F perfiuorocyclohexane C F andperfluoroheptane C7F16.

Example 14 A mixture was made of purified C7F14 (0.005 hydrogen content)and perfluorocyclohexene 0.002% hydrogen content) in a ratio 20: 1. Themixture was sealed with chlorine and oxygen (Cl :O =1:50) to a pressureof 1.7 atmos. and ultraviolet radiation was allowed to fall on themixture for 2 days. The liquid products were washed with dilute aqueousalkali and distilled. Only perfluoromethylcyclohexane was recovered.Examination of the aqueous alkaline solution revealed the presence ofperfluoro-acids of varying chain lengths, but these were 'not examinedfurther.

Example 15 Crude perfluorodimethylcyclohexane 10 g.) containing 0.015%hydrogen was sealed in a silica tube with oxygen (3 atmos.) and exposedto ultraviolet light for 2 days. The hydrogen content had been reducedto 0.012%. After further periods of 4 and 10 days the hydrogen contentwas 0.009 and 0.006%

Under comparable conditions a similar mixture to which chlorine (0.5 g.)had been added gave a product containing 0.008% after 2 days and 0.006%after a further 4 days. This shows that chlorine speeds up thepurification reaction but is not essential since purification canproceed without it.

Example 16 A perfluorocarbon oil (3.0 g.) B.P. -180/10 mm. containing0.020% hydrogen was sealed with fluorine (0.2 g.) and oxygen (1.5atmos.) and irradiated for 3 days with ultraviolet light. Afterdistillation the oil contained less than 0.005% hydrogen.

A parallel experiment using chlorine instead of fluorine as initiatorgave a chlorine-free product containing 0.008% hydrogen after a similarperiod.

Example 17 Crude C F from the vapor phase fiuorination of naphthalenecontained 0.05 hydrogen. It was passed through a platinum tube heatedelectrically to 350400 together with a mixture of chlorine and oxygenThe flow rate was adjusted so that pyrolytic oxidation of pure C Ffailed to occur. The condensed product was washed and distilled to givea purified product containing 0.007% hydrogen.

A similar experiment using fluorine as initiator gave a productcontaining 0.005 hydrogen. An autoclave may be used instead of the flowmethod if prolonged contact is desired.

9 Example 18 Example 19 Repetition of Example 18 under the followingconclitions gives perfluorodimethylcyclohexane of the purity indicated:

Contact Percent H Temp, C. 01 Time in Purified Material 500-550 1:5 25sec 0. 004 600-650 1:5 7 sec 0.006 350400 1-10 8 0.015

Example 20 Crude perfluoromethylcyclohexane containing 0.07% hydrogen ispassed through a platinum tube heated electrically to 450500 togetherwith a mixture of fluorine and oxygen (P 0 1270). The contact time of 20sec. is such that pyrolytic oxidation of pure hydrogen-freeperfluoromethylcyclohexane is negligible. The condensed, washed anddistilled product contains 0.005% hydrogen.

I claim:

1. A method of purifying a saturated perhalocarbon in which the halogensubstituents are selected from the group consisting of fluorine andchlorine and the atomic ratio of fluorine to chlorine is at least one,said perhalocarbon containing impurities selected from the groupconsisting of (1) hydrogen-containing compounds of structure R-H whereinR is selected from the group consisting of perfluorocarbons,perfluorochlorocarbons, polyfluorohydrocarbons andpolyfluorochlorohydrocarbons, and (2) compounds selected from the groupconsisting of olefinically unsaturated perfluorocarbons,perfluorochlorocarbons, polyfluorohydrocarbons andpolyfluorochlorohydrocarbons, comprising subsequent subjecting suchperhalocarbon to an energy source consisting essentially of nonradiantheat at a temperature in the range from 350 to 700 C. in the presence ofoxygen and an initiator selected from the group consisting of fluorine,chlorine, bromine and interhalogen compounds consisting of iodine andchlorine, iodine and bromine and chlorine, bromine and fluorine, andchlorine nad fluorine for a time suflicient to oxidize the major part ofthe impurities to form products easily removable from said perhalocarbonbut insuflicient to cause degradation of a significant amount of saidperhalocarbon.

2. The method of claim 1 in which the perhalocarbon is aper-fluorocarbon.

3. The method of claim 1 in which the oxygen is present in an amount atleast five times that of the initiator.

4. The method of claim 3 in which the initiator is chlorine.

5. The method of claim 3 in which the initiator is a halogen fluoride.

6. The method of claim 3 in which the initiator is fluorine.

References Cited by the Examiner UNITED STATES PATENTS 2,472,946 6/1949Hart et al 260544 2,639,301 5/1953 Rich et al. 260653.8 2,712,554 7/1955Miller 260544 2,719,171 9/1955 Kalb 260486 OTHER REFERENCES Francis etal., Journal of the Chemical Society (London) (1955), pp. 2151-2155.

LEON ZIVER, Primary Examiner. ALPHONSO D. SULLIVAN, Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,218,363 November 16, 1965 Robert N. Haszeldine It is hereby certifiedthat error appears in the above numbered patent requiring correction andthat the said Letters Patent should read as corrected below.

Column 1, line 66, for "impurities" read impurities column 2, line 4,for "hake" read make line 37, for "it lost" read is lost column 4, line16, for "perhocarbon" read perhalocarbon column 9, in the table, thirdcolumn, line 3 thereof, for "8" read 8 min. column 10, line 11, after"bromine", first occurrence, insert bromine line 12, for "nad" read andline 39, for "LEON ZIVE 'R" read EON ITVER Signed and sealed this 16thday of August 1966.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner ofPatents

1. A METHOD OF PURIFYING A SATURATED PERHALOCARBON IN WHICH THE HALOGENSUBSTTITUENTS ARE SELECTED FROM THE GROUP CONSISTING OF FLUORINE ANDCHLORINE AND THE ATOMIC RATIO OF FLUROINE TO CHLORINE IS AT LEAST ONE,SAID PERHALOCARBON CONTAINING IMPURITEIS SELECTED FROM THE GROUPCONSISTING OF (1) HYDROGEN-CONTAINING COMPOUND OF STRUCTURE R-H WHEREINR IS SELECTED FROM THE GROUP CONSISTING OF PERFLUOROCRABONS,PERFLUOROCHLOROCARBONS, POLYFLUOROHYDROCARBONS ANDPOLYFLUOROCHLOROHYDROCARBONS, AND (2) COMPOUNDS SELECTED FROM THE GROUPCONSISTING OF OLEFINICALLY UNSATURATED PERFLUOROCARBONS,PERFLURORCHLOROCARBONS, POLYFLURORHYDROCARBONS ANDPOLYFLURORCHLOROHYDROCARBONS, COMPRISING SUBSEQUENT SUBJECTING SUCHPERHALOCARBON TO AN ENERGY SOURCE CONSISTING ESSENTIALLY OF NONRADIANTHEAT AT A TEMPERATURE IN THE RANGE FROM 350* TO 700*C. IN THE PRESENCEOF OXYGEN AND AN INITIATOR SELECTED FROM THE GROUP CONSISTING OFFLUORINE, CHLORINE, BROMINE AND INTERHALOGEN COMPOUNDS CONSISTING OFIODINE AND CHLORINE, IODINE AND BROMINE AND CHLORINE, BROMINE ANDFLUORINE,AND CHLORINE AND FLUORINE FOR A TIME SUFFICIENT TO OXIDIZE THEMAJOR PART OF THE IMPURITIES TO FORM PRODUCTS EASILY REMOVABLE FROM SAIDPRHALOCARBON BUT INSUFFICIENT TO CAUSE DEGRADATION OF A SIGNIFICANTAMOUNT OF SAID PERHALOCARBON.